[Extractor] Improve commandline debugging, forward stderr

Summary:
In case baloo_file is started from an interactive shell/tty, the debug
output no longer goes to the syslog/journal, but to stderr. Make sure
stderr does not end up in the stdin of the parent process but is forwarded.

Test Plan:
start baloo_file from the shell, with debugging enabled:
QT_LOGGING_RULES='org.kde.baloo=true' PATH=./bin ./bin/baloo_file
-> debug output from the extractor appears on stderr

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16267